By Eric Kandel November 29, 2020 ⋅ 3 min read ⋅ Books
General Notes
New science of mind; communicating changes.
Cajal’s four principles of neural organization
The neuron. A nerve cell is a neuron, the elementary signaling unit of the nervous system.
The synapse. The spot where neurons communicate with each other.
Connection specificity: a given neuron will only communicate with specific cells and not with others.
Dynamic polarization: within a neuron, signals travel in only one direction. This principle allows one to determine how information flows in neural circuits.
The single function of a neuron is to signal. To signal what though? Changes.
Synapses are one-way because the machinery for neurotransmitter release is located only in the presynaptic neuron.
Almost all inhibitory neurons are interneurons.
Inhibitory neurons select among competing signals.
Electrical signaling represents the language of mind and listening in on those conversations.
Neurons are different from wires in that wires conduct electrical signals at near the speed of light but the signal deteriorates badly over long distances.
Neurons conduct signals much slower but the signal is regenerated along the way. Nerves sacrifice speed of conduction for active propagation.
All APs are pretty much the same. Coding intensity results from the frequency of APs. Coding duration results from the duration of APs. What accounts for the differences in information carried by neurons? Anatomy.
The fundamental task of a nerve cell is integration.
Selecting an anatomically simple system is crucial to the success of an experiment. The choice of an experimental system is one of the most important decisions a biologist makes.
Memory is a distinct function. Short and long term memory are stored separately.
Loss of medial temporal structures destroys the ability to convert new short term memories into long term memories.
Long term memory is stored in the cerebral cortex. It’s stored in the same area that originally processed that information.
The cellular mechanisms of learning and memory reside not in the special properties of the neuron itself, but in the connections it receives and makes.
The strength of synapses can be changed by applying different patterns of stimulation.
E.g. Habituation, sensitization, and classical conditioning.
Spacing the training with rest enhances the ability to establish long-term memory.
In Aplysia, the neural architecture of behaviour is invariant but this isn’t completely conserved in humans.
Weaker synapses mean weaker connection or weaker/unimportant change communicated by that pathway while vice versa is also true.
Genetics and developmental processes specify the connections among neurons, but they don’t specify the strength of those connections. The strength is regulated by experience.
Learning selects among a large repertoire of preexisting connections and alters the strength of a subset of those connections.
A prerequisite for studying behavioural modification is the analysis of the wiring diagram underlying the behavior, the connectome. Once the wiring diagram of a behavior is known, the analysis of its modification becomes greatly simplified.
One fundamental feature of memory is that its formed in stages. The early phases of memory storage are dynamic and sensitive to disruption. Long-term memory requires the synthesis of new proteins as found by the drug experiment done by Louis Flexner.
The number of synapses in the brain changes with learning. Long-term memory persists for as long as the anatomical changes are maintained.
To be useful, a memory has to be recalled.
Two kinds of neural circuits important for behaviour and learning: mediating circuits and modulating circuits.
Mediating circuits produce behavior directly and act like a student, while modulating circuits fine-tunes the behavior in response to learning and acts like a teacher.
The changes to a synapses are mostly one-sided, the presynaptic side releases more neurotransmitter.
Serotonin, a modulatory molecule, increases the production of cyclic AMP in the presynaptic terminal, which lasts as long as the slow synaptic potential, which increases the synaptic strength between sensory and motor neurons.
The slow synaptic potential is caused by the closing of a serotonin-sensitive potassium channel. When the channel is closed, ions move out of the cell less rapidly, increasing the duration of the AP by slowing the descending stroke.
Slowing the AP allows more time for calcium to flow into the presynaptic terminal, allowing more neurotransmitter to be released.
Cyclic AMP and protein kinase A also act directly on the machinery that releases synaptic vesicles, further stimulating the release of neurotransmitter. This explains the neurophysiology of short-term memory.
Does the modulatory system changes as well? Does it modulate itself or does another system modulate it? How are short-term changes converted into long-term memories?
Long-term memory formation depends on the synthesis of new proteins. Even though a neuron can make thousands of synaptic connections, individual synapses can be modified independently.
Genes must be switched on to form long-term memory.
The growth and maintenance of new synaptic terminals makes memory persistent.
Sensation is an abstraction, not a replication, of the real world.
There is no single cortical area to which all other cortical areas report exclusively, either in the visual or in any other system.
In sum, the cortex must be using a different strategy for generating the integrated visual images.
Consciousness is awareness of being aware. Crick believes that the secret of consciousness lies in the claustrum.